This invention relates to printing plates, and in particular, a printing plate adapted for self-locking to the plate cylinder of a press apparatus.
Offset printing presses, for example, employ printing cylinders having thin, metal plates attached to them. The cylinders conventionally have an axial groove in their outer surface that extends radially within the cylinder. Commonly, the printing plates are prepared for positioning on the cylinder by preforming bends on the plates, for example, along a leading edge and a trailing edge of the plate, to facilitate their attachment. After bending, the plates are wrapped around the cylinder periphery by attaching the leading edge of the plate to one edge defining the axial groove in the cylinder, and thereafter wrapping the plate around the cylinder until the trailing edge of the plate enters the groove at the opposite, second edge defining the axial groove in the cylinder. Conventionally, two plates are required to cover the periphery of the cylinder.
A number of arrangements have been proposed for holding the plates in position on the cylinders. In general, these arrangements have employed the use of an oversized printing plate, that is to say, oversized with respect to the peripheral dimension of the printing cylinder and some cam operated latching mechanism which engages at least one of the leading and trailing edges of the plate to tension the plate in the running position of the cylinder. Examples of this form of construction are shown in the U.S. Pat. 3,626,848 to Tafel and Etchell, U.S. Pat. No. 3,757,691.
Where the plates have not been oversized, they still have required complicated cam arrangements to secure the plates to the cylinder, as for example, shown in the U.S. Pat. No. 3,608,487 to Leuhrs.
While these various examples of plate and plate tensioning assemblies work for their intended purpose, they all have shortcomings, economic or otherwise, which become apparent either during manufacture or operation of the associated printing press. For example, the tensioning type of construction as described in the Tafel U.S. Pat. No. 3,626,848, in which the fingers engage at least an end of the printing plate, causes that end of the printing plate engaged by the tensioning device to be drawn away from the supporting structure of the cylinder. This occurs in the area of the groove or slot extending into the radial depth of the cylinder so that the printing plate is unsupported along that groove. The plate cylinder, in operational use, contacts ink rollers, a water form roller and a blanket cylinder, which under some pressure, applies ink and water to the plate and permits the transfer of the desired information from the plate cylinder to the blanket cylinder. As this operation occurs, the various rollers tend to force the unsupported portion of the printing plate inwardly of the groove in the plate cylinder, flexing the printing plate each time the ink and water rollers crosses a groove in the plate cylinder. This flexing eventually weakens the material from which the plate is constructed, with the result that the plates crack and tensioning grip is lost on the plates. Eventually, the plate separates from the plate cylinder. Plate separation causes press down time, and results in the loss of web material. Often, plate separation also damages the relatively expensive printing blanket carried by the blanket cylinder.
To guard against the possibility of plate breakage, it has been common to use relatively thick printing plates, in the order of twelve thousandths thicknesses and above, for the plates. While thicker plates tend to reduce the occurence of plate breakage, it has not heretofore prevented such plate breakage and the loss attended with that breakage.
We have found that the cause of plate breakage is a result of the fact that many plates are oversized with respect to the printing cylinder, and that oversize has resulted in the above described plate deterioration.
We further have found that the breakage problems inherent with prior art plate designs may be eliminated, and that a thinner plate material may be employed in the plates when the plate is sized to fit the plate cylinder snugly and is formed with a special reverse bend on one of the edges of the plate. The spacing of two positioning bends in the plate is chosen so that the plate must be forced over the edges of the axial groove in the cylinder. That is, the plate actually must be snapped into position. A reverse bend outboard of one of the positioning bends facilitates application of the plate to plate cylinders and helps maintain the tension of the plate on the plate cylinder so that the plate remains in position, even if some elongation of the plate occurs during use. Surprisingly, plate elongation has not been found to be a problem when the reverse bend is used in conjunction with the positioning bends arranged so that the plate must snap on the plate cylinder. The plate also is compatible with a wide range of press types, and while the locking mechanisms of the prior art are not required to hold the plate in position on the press, the plate of this invention may be employed with pressed employing such locking mechanisms. When so employed, however, the locking mechanisms are engaged in the reverse sense of their intended function. Such prior art locking mechanisms commonly have cam operated fingers arranged to engage one end of the plate. The fingers, under cam control, have a first plate insertion position and a second plate locking position. When the plate of our invention is employed, the prior plate locking position of the fingers becomes the plate insertion position, while the prior plate insertion position becomes the plate locking position.
In use, the plate of this invention has resulted in considerable material cost savings in that a plate thickness of less than twelve thousandths of an inch, and having a length dimension shorter than prior art plates may be employed. Plate thicknesses in the range of eight to nine thousandths consistently have been employed without adverse affect. Plate breakage in normal use with presses employing our plate have been nonexistent. In one application, the plates are employed for printing newspapers for a large metropolitan newspaper. In that application, use of the plate of this invention has resulted in an estimated one hundred thousand dollar a year savings in material cost alone. Although press down time savings have not been calculated, plate breakage has been deminimis. Consequently, additional savings from reduced waste and lower labor costs are expected.
One of the objects of this invention is to provide a low cost plate structure for printing cylinders.
Another object of this invention is to provide a snap-on, self-locking plate for plate cylinders.
Another object of this invention is to provide a plate for printing cylinders that is compatible with existing cylinder designs.
Yet another object of this invention is to provide a plate cylinder construction which may employ plates having thicknesses less than twelve thousandths of an inch.
Still another object of this invention is to provide a plate cylinder, the surface area of which has two positioning bends formed in it so as to define a leading edge, and a trailing edge, the trailing edge having a reverse bend formed in it out-board of the positioning bend.
Other objects will be apparent to those skilled in the art in light of the following description and accompanying drawings.